Current transformer



April 3, 1934. E. TRrrscHLER 1,953519 CURRENT TRANSFORMER med oct. 17. 1931 v v v v v AAAAAAAAA Av/v/v/v/A/v/Y/L V a ig/1 /vw/a/vom gm Patened Apr. 3, 1934 UNITED sTATEs PATENT OFFICE to Aktiengesellschaft Brown Boveri & Cie.,

Baden, Switzerland, a joint-stock company of Switzerland Application October 17, 1931, Serial No. 569,453

In Germany October 17, 1930 Claims.

This inventionvrelates to improvements in electric current transformers, and particularly to transformer structures for use in systems in which a plurality of transformers must be so accurate in operation as to havev substantially the same characteristics.

When electric current transformers are produced by quantity production methods, it is practically impossble to obtain a number of transformers having substantially the same characteristics as regards either transformation ratio or phase angle and it is necessary to use a correction factor for such transformers when a number of transformers are to be used for similar purposes in the same system. When it becomes necessary to obtain a number of transformers having the same Operating characteristics from a lot of transformers produced by quantity production methods, it is necessary to test a very large number of transformers to obtain even a few transformers having sufiiciently similar characteristics for use for similar purposes. When current transformers are to be used for a purpose such as diiferential protection of an alternating current line, it is particularly necessary that the transformers have the same characteristic curves. Small differences in either the core or the winding losses Fr in the arrangement of the windings may produce curves which extend parallel to each other, but do not coincide. Such deviations from coincidence are, however, generally so slight that a change in the windings is sufiicient for most purposes. For differential protection, or any other use in which the transformers must have exactly the same characteristics, a change in the windings, however, causes too great a change to obtain the necessary coincidence in operation to secure sufficiently sensitive operation of the differential protective device.

Various solutions of the above problem have been proposed, such as the provision of an aperture through the core through which aperture the last turn of a winding threaded thereby enclosing only a portion of the core. The use of magnetic shunts or flux shields has likewise been proposed. But none of the above means fully accomplished the desired result for the reason that such structure weakened the core cross-section,

endangered the windings at the apertured point, and did not permit a suficiently exact voluntary equalization of the diiferences unavoidable in manufacture.

It is, therefore, among the objects of the present invention to provide a current transformer in which very fine adjustments may be made in the Operating characteristics.

Another object of the invention is to provide an electric current transformer in which adjustments of the Operating characteristics less than the change produced upon change of the windings may be obtained.

Another object of the invention is to provide an electric current transformer in which the characteristic Operating curves may be made to coincide.

Objects and advantages other than those above set forth will be apparent from the following description when read in connection with the accompanying drawing in which:

Figure 1 diagrammatically illustrates an electric current transformer provided With means for adjusting the ratio and/or phase angle thereof; and

Fig. 2 is a cross section on the line II-II of the core shown in Fig. 1 and showing the means for adjusting the ratio and/or phase angle shown in Fg. 1.

Referring more specifically to the drawing by characters of reference, the reference numeral 3 designates a transformer core which is built up of the usual iron sheet metal laminations and which has a primary winding 4 and a secondary winding 5 arranged thereon. A sheet '7 of diamagnetic material such as copper is inserted between the laminations of the core 3 with the ends thereof extending beyond the sides of the core. The diamagnetic sheet may be inserted through the core at any point, but preferably is passed through the core at a point which is free from windings of any description. The portions of the sheet 7 extending beyond the outer dimensions of the core are preferably of such length that they may be bent about a portion of the core and the ends of the sheet are connected with a variable resistance 8 by means of which'the action of the sheet 7 on the magnetic flux in the core may be adjusted by movement of the sliding contact 9.

The use of the diamagnetic sheet 7 so controls the permeability of the core 3 that the flux density therein remains constant and the ratio of transformation therefore also remains constant regardless of the load on the transformer. Any error in phase angle may be corrected by the use of the adjustable resistance 8, which in effect makes the sheet 7 a short circuited turn on the core. An exact adjustment to a given set of characteristics, of any number of transformers, may thus be obtained and any number of transformers may be adJusted to the same characteristics even though constructed by quantity production methods. Such transformers may, therefore, be used for any purpose requiring extreme accuracy, such as differential protection in which a number of transformers must have coinciding characteristic curves.

Although but one embodiment of the present invention has been illustrated and described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention or the scope of the appended claims.

lThe invention claimed is:

1. A current transformer comprising a core of magnetically permeable material formed as a stack of laminations arranged in side by side relation, a primary winding, a secondary winding, asheet odiamagnetic material extending parallel to and supported between the sides of adjacent ones of said laminations, and a resistance connected in circuit with said sheet to form a tertiary circuit operable to adjust the phase angle of current delivered by said secondary winding relative to current flowing'in said primary winding and to adjust the rat'; of transformation of said windings.

2. A current transformer comprising a core of magnetically permeable material formed as a Stack of lamlnations arranged in side by side relation, a primary winding, a secondary windrelation, a primary winding, a secondary wind-- ing, a sheet of diamagnetic electrically conductive material extendinz through said core parallel to and supported between the sides of adjacent ones of said laminations, the said sheet being formed to enclose a portion of said core and being operable to adjust the electro-magnetna dimensions of the transformer.

4. A current transformer comprising a primary winding and a secondary windlnz, a core for said windings comprising a stack of mag-I' netically permeable laminations, and a tertiary winding comprising a oonductive member projecting through said core and supported between adjacent of said laminations and also comprising a resistance ldevice connected across the projecting portions of said conductive member.

EUGEN TRITSCHIIR.

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